The present invention relates to the field of synthetic aperture radar image processing. More specifically, the present invention relates to the field of coherent change subtraction of mission and reference synthetic aperture radar data.
Synthetic aperture radar (SAR) data is often subjected to analysis by backscatter amplitude or phase change detection, e.g., for the detection of a moving target. Similarly, SAR data is often subjected to analysis by combined noise and backscatter energy suppression, e.g., for low clutter-to-noise applications. Conventional techniques for analysis of SAR data include interferometric change detection, displaced phase center moving-target identification (MTI), and space-time adaptive processing (STAP).
Interferometric change detection identifies change when the detector output is small. The detector is a small local correlation window typically ranging in size from 3xc3x973 pixels to 15xc3x9715 pixels. To maximize correlation, the phase history aperture in both range and azimuth is trimmed for some common ground plane. Spatially variant registration is required to correct for changes in the layover due to different geometries. Detection occurs when the correlation is low.
Problems exist with interferometric change detection. Interferometric change detection is a biased estimator and also decreases dynamic range. This results in a decrease in overall sensitivity. The ability of interferometric change detection to detect small changes is therefore limited. In addition, interferometric change detection is prone to false alarms from specular scatterers, and has decreased reliability with time varying waveforms.
Displaced phase center MTI uses coherent subtraction to suppress the ground clutter return. Moving targets, which are not suppressed by this process and can be detected by a conventional constant false alarm rate (CFAR) detection routine.
Problems exist with displaced phase center MTI. Displaced phase center MTI does not use full azimuth resolution. Also, displaced phase center MTI does not trim away non-correlating energy. These features result in decreased overall sensitivity. In addition, displaced phase center MTI has decreased reliability with time varying waveforms.
STAP is a locally adaptive technique that minimizes total competing energy. Moving targets, which are not suppressed by this process, can now be detected. STAP provides good performance for suppression of specular scatterers and jammers. Suppression of energy from diffuse scatterers is not as good.
Problems exist with STAP. STAP uses null casting and fails to compensate for all sources of distortion. This limits clutter suppression.
Accordingly, it is an advantage of the present invention that a method for coherent change subtraction of synthetic aperture radar (SAR) data is provided.
It is another advantage of the present invention that dynamic range is increased.
It is another advantage of the present invention that detection of small changes is enhanced.
It is another advantage of the present invention that false alarms from specular scatterers are suppressed.
It is another advantage of the present invention that the method works well with time varying waveforms.
It is another advantage of the present invention that non-correlating energy is trimmed off.
The above and other advantages of the present invention are carried out in one form by a method for coherent change subtraction of mission and reference SAR data. Complex images are formed of both the mission and reference SAR data. The complex images are integrated to form an integrated complex image. The integrated complex image is divided into at least one patch containing mission-image and reference-image data, where the mission-image data is data from the mission complex image and the reference-image data is data from the reference complex image. An interferogram is formed of each patch. A composite interferogram is produced. The mission complex image is subtracted from the reference complex image to form a delta complex image. The delta complex image is then postprocessed.